1. Field of the Invention
The present invention relates to an air-fuel ratio control device for an internal combustion engine, which device carries out an air-fuel ratio feed-back control using a first air-fuel ratio detector arranged upstream of a catalytic converter, and a second air-fuel ratio detector arranged downstream thereof, in the exhaust system.
2. Description of the Related Art
An exhaust system of an internal combustion engine is usually provided with a three-way catalytic converter which oxidizes CO and HC, and deoxidizes NO.sub.x so that these three harmful materials in the exhaust gas are converted into harmless materials, as CO.sub.2, H.sub.2 O, and N.sub.2. The purifying ability of the three-way catalyst depends on an air-fuel ratio of the mixture in an engine cylinder, and it is known that when the air-fuel ratio is stoichiometric, the three-way catalyst can purify all of these three harmful materials at the same time. To counter a variation of the air-fuel ratio, the three-way catalyst usually has an O.sub.2 storage ability such that it absorbs and stores excess oxygen existing in the exhaust gas when the mixture is on the lean side, and it releases oxygen when the mixture is on the rich side.
An air-fuel ratio control device which has a first air-fuel ratio detector arranged upstream of the catalytic converter and a second air-fuel ratio detector arranged downstream thereof in the exhaust system, is known. The device corrects a target amount of injected fuel, determined from the current engine operating condition, on the basis of a difference between the air-fuel ratio upstream of the catalytic converter detected by the first air-fuel ratio detector and the stoichiometric air-fuel ratio.
In the air-fuel ratio control device, a standard output of the first air-fuel ratio detector corresponding to the stoichiometric air-fuel ratio is corrected, on the basis of a difference between the air-fuel ratio downstream of the catalytic converter detected by the second air-fuel ratio detector and the stoichiometric air-fuel ratio.
In such air-fuel ratio control device, the air-fuel ratio downstream of the catalytic converter usually varies only within a relative small range due to the O.sub.2 storage ability of the catalytic converter. However, once the catalytic converter deteriorates and thus the O.sub.2 storage ability thereof drops, the air-fuel ratio downstream of the catalytic converter begins to vary within a relative large range as does the air-fuel ratio upstream thereof. When the standard output of the first air-fuel ratio detector is corrected as the above-mentioned, a correction value is determined on the assumption that the air-fuel ratio detected by the second air-fuel ratio detector varies only within the small range. Accordingly, once the catalytic converter deteriorates, the correction value is made relatively large in spite of a normal variation of the air-fuel ratio upstream of the catalytic converter and thus hunting of the air-fuel ratio in the engine cylinder can be caused.
Japanese Unexamined Patent Publication No. 3-217634 discloses an air-fuel ratio control device which makes the air-fuel ratio vary with the stoichiometric air-fuel ratio as a center line, on the basis of the output of the first air-fuel ratio detector. To be concrete, the device makes a correction factor for correcting an amount of injected fuel increase by a rich skip amount and then gradually increase by a rich integration amount when an output of the first air-fuel ratio detector has changed from the rich side to the lean side. The device makes the correction factor decrease by a lean skip amount and then gradually decrease by a lean integration amount when an output of the first air-fuel ratio detector has changed from the lean side to the rich side. The device makes the skip amounts or the integration amounts change to correct the standard output of the first air-fuel ratio detector, on the basis of the output of the second air-fuel ratio detector. Moreover, the device comprises detection means for detecting a deterioration of the catalytic converter. The device forces the skip amounts or the integration amounts become small when the catalytic converter deteriorates, to prevent hunting of the air-fuel ratio.
However, when the catalytic converter deteriorates, the standard output of the first air-fuel ratio detector is not corrected precisely so that combustion deteriorates. Moreover, the O.sub.2 storage ability of the catalytic converter also drops when the catalyst is not activated. Accordingly, when the catalyst is not activated, the device does not forces the skip amounts or the integration amounts to become small so that hunting of the air-fuel ratio can still be caused.